The present invention relates to a vacuum valve used in an apparatus such as vacuum circuit breaker and the like, and in particular to a technology that can effectively be applied to joining of bellows incorporated in a vacuum valve.
For example, a vacuum switch using a vacuum valve as a device to break a circuit carrying relatively heavy electric current, is known.
Japanese Patent No. 3369366, for example, discloses a structure comprising a hermetically sealed insulative vessel, the interior of which is maintained at a predetermined degree of vacuum, a contact point consisting of a fixed electrode anchored at a tip of a fixed current conduction rod and a movable electrode anchored at a tip of a movable current conduction rod. The contact is disposed in the insulative vessel. A bellows forms a part of the insulative vessel where the movable current conduction rod is inserted. One end of the bellows is hermetically fixed to the insulative vessel and the other end is hermetically fixed to the movable current conduction rod. Expansion and contraction of the bellows in the axial direction allows the movable electrode (i.e. the movable current conduction rod) to be displace in the axial direction while maintaining the vacuum in the insulative vessel. Switching of the current conduction circuit connecting to the fixed current conduction rod and the movable current conduction rod is performed by contacting and separating the fixed electrode and the movable electrode.
If the degree of vacuum is impaired in a vacuum valve having a structure as describe above, are discharge occurs between the electrodes to inhibit current switching operation of the circuit. Therefore, it is essential from the aspect of reliability of operation and to prolong the working life of the arrangement to ensure, the air tightness of the wall of the bellows itself, which expands and contracts, and at the joint between the insulative vessel and the bellows.
Consequently, the bellows is one of the most important parts of a vacuum valve. Austenitic stainless steel is generally used as the material for the bellows because of its good corrosion resistance. TIG (Tungsten-Inert-Gas) welding or silver soldering can be employed for joining ends of the bellows to an end plate and a movable current conducting rod.
TiG welding, however, involves problems that dimensions of a part relative to another part must be strictly controlled and that the quality of the joint may happen to be poor depending on welding skill of the worker.
While silver soldering does not have the above-mentioned problem as TIG welding, attention must be paid to the surface characteristic of the article to be soldered. For, example, austenitic stainless steel, a possible material for bellows, can have an oxide film on the surface thereof, and as result, does not exhibit satisfactory wettability with the silver solder due to this oxide film, irrespective that it resides in excellent corrosion resistance.
Consequently, the surface of bellows is normally plated with nickel in order to secure wettability of silver solder. Japanese Patent Unexamined Publication No. 2004-79446 discloses nickel plating on a solder joint of bellows.
However, the nickel plating on a bellows, which generally has a small thickness and a complicated shape, cannot be carried out using a batch process such as barrel plating, which allows many bellows to be processed at one time, in a manner which prevents imperfections such as pits. Consequently, it is unavoidable to use an individual plating process such as rack plating, which takes much time and work. Therefore, the plating itself, which is a prerequisite for employing soldering, leads to considerably high cost.
A bellows that is an object of silver soldering can be a so-called seamless type bellows as describe in Japanese Patent Unexamined Publication No. 2001-6503.
A seamless type bellows is normally manufactured using a procedure wherein a disk is punched by blanking from a sheet having a thickness of about 0.5 mm. The disk is subjected to a procedure including repeated steps of cleaning, lubricating, deep drawing, cleaning, annealing, lubricating, deep drawing, and so on, to obtain a deep cylindrical cup having a pipe-shaped side wall about 0.1 mm thick and a bottom plate (enclosing end of the cup) relatively thick (a thickness of the raw sheet i.e. about 0.5 mm). This cup is subjected to hydraulic pressure on the inner wall thereof forming ribs to obtain a bellows.
As described above, a bellows of the seamless type is manufactured through a rather complicated multiple steps, so it is considerably expensive, which is problematic.
To incorporate this seamless type bellows having a bottom plate into a vacuum valve, as described in the Japanese Patent Unexamined Publication No. 2001-6503 mentioned previously, a through hole for an electrode is bored at the bottom, and the bellows, a bellows shielding member, and a binding ring are stacked and fixed by soldering. This integrating structure is necessary but rather complicated.
The complexity of the integrating structure may cause insufficient filling of the solder in the gaps between the bellows, the bellows shielding member, and the binding ring. Thus, there is concern with respect to degradation of strength and air tightness at the solder joint.
In light of the above problems, an object of the present invention is to attain low cost in a vacuum valve including a soldering structure in a movable portion of a bellows.
Another object of the invention is to shorten the manufacturing period for a vacuum valve including a soldering structure in a movable portion of a bellows.
Still another object of the invention is to improve strength and air tightness of the joint in the soldering structure in a movable portion of a bellows in a vacuum valve.